The "problem" is typically PHP is intended to run as module in a webserver. You may need to install the commandline version of php before you can run php scripts from the commandline:
apt-get install php5-cli
/usr/bin/php is default location for the php binary to be placed, but if you for instance compile php from source it may be somewhere else.
Typically PHP scripts aren't formatted as shell scripts, so you need to tell cron which interpreter should be used to execute the php script; that's the reason to use the commandline
You could format you script with the shebang and make it executable (
chmod +x script.php) and then you can call it directly from the commandline, without specifying php as the interpreter ( i.e.
print "Hello world!\n" ;